1. Field of the Invention
The present invention relates to a base station, a mobile station, a wireless communication system, and a method for controlling the base station and the mobile station, and particularly relates to a technology for transmitting information from each mobile station to the base station via a radio channel shared among a plurality of mobile stations (hereinafter may be referred to as the shared channel).
2. Description of the Related Art
Cited as an applied example of the above information transmission technology is a wireless communication system that collects location information of a plurality of mobile stations on a base station side. The wireless communication system is suitable for a use for displaying and monitoring the location of each mobile station on a map.
Generally, the location information is collected as each one of the mobile stations transmits its location information acquired from GPS (Global Positioning System) to the base station via the shared channel.
Here, in order to avoid contention over the transmission timing of the location information among the mobile stations, one of the following two modes is adopted in many cases as the transmission mode of the location information.
Transmission Mode 1
This mode is, generally, that the mobile station transmits the location information at the request of the base station. Specifically, the base station transmits a polling signal sequentially to all active mobile stations to request their location information. Each one of the mobile stations replies to the base station with the location information as a response to the polling signal.
Transmission Mode 2
This mode is, generally, that the mobile station autonomously transmits the location information to the base station. Specifically, the mobile station periodically transmits the location information to the base station in accordance with a preset transmission interval and time mark, considering the maximum number of mobile stations that can be accommodated by the base station, the number of traffic channels, and the like. Here, the transmission interval indicates the time interval during which the mobile station needs to transmit the location information. Moreover, the time mark indicates the transmission timing of the location information that is different from one another among the mobile stations, and an offset time value from UTC (Universal Time Coordinated) or an identifier thereof is set therefore.
It is desirable to set the frequency of transmission of the location information by the mobile station as high as possible to appropriately monitor the location of the mobile station in the above wireless communication system. To put it another way, it is necessary to improve efficiency in information transmission from the mobile station to the base station.
However, there is a problem in the transmission modes 1 and 2 that efficiency in information transmission is low.
Specifically, in the transmission mode 1, a polling signal from the base station to the mobile station oppresses the band of the shared channel. Therefore, it is not possible to allocate a sufficient time to transmit information to each one of the mobile stations, and efficiency in information transmission decreases (theoretically, the transmission mode 1 requires communication time twice as long as time required by the transmission mode 2). As a reference technology, Japanese Laid-open Patent Publication No. 06-29897 discloses a wireless communication system where mobile stations are divided into a plurality of groups, and a base station polls the mobile stations sequentially in the group on a group basis. However, if all the active mobile stations are requested to transmit information, polling signals oppresses the band of the shared channel similarly to the transmission mode 1, and again efficiency in information transmission decreases.
On the other hand, in the transmission mode 2, the transmission interval is set in accordance with the maximum number of mobile stations that can be accommodated by the base station. Therefore, depending on the operating status of the mobile station, efficiency in information transmission may decrease. For example, assume that the maximum number of mobile stations that can be accommodated by the base station is “180”, and a time necessary for the mobile station to transmit the location information is “one second”. In this case, the transmission interval is set to “180 seconds”. However, the active mobile stations need to transmit the location information at intervals of “180 seconds” under the condition that a part of mobile stations are active. To put it another way, many idle periods when no information is transmitted on the shared channel may occur.